1. Field of the Invention
The present invention relates to a single-sided circuit board and a manufacturing method therefor, and more particularly to a single-sided circuit board used for manufacture of a multilayer printed circuit board having an interstitial via hole (IVH) structure and a manufacturing method therefor.
2. Description of the Related Art
A conventional multilayer printed circuit board is constituted by a laminate obtained by alternately laminating copper-clad laminates and prepregs. The laminate has a surface on which a surface wiring pattern has been formed. Moreover, an internal wiring pattern is formed between interlayer insulating layers. Through holes are, by punching formed in the direction of the thickness of the laminate to establish the electric connections among internal wiring patterns or among internal wiring patterns and the surface wiring patterns.
The multilayer printed circuit board having the above-mentioned through hole structure must have regions for forming the through holes. Therefore, the density at which elements are mounted cannot be easily raised. As a result, there arises a problem in that the multilayer printed circuit board having the through hole structure cannot easily address needs for considerable reductions in the sizes of the portable electronic apparatuses, realizing narrow pitch packages and practical use of MCM.
As an alternative to the foregoing multilayer printed circuit board having the through hole structure, a multilayer printed circuit board has recently received attention which is formed into a full-thickness interstitial via-hole (IVH) structure which is able to easily address raising of the density.
The multilayer printed circuit board having the full-thickness IVH structure is a printed circuit board having a structure in which via holes for electrically connecting conductive layers to one another are formed in each of the interlayer insulating layers which constitute the laminate. That is, the foregoing printed circuit board has the via holes (buried via holes or blind via holes) which do not penetrate the substrate on which the circuit is formed and which electrically connect the internal wiring patterns to one another or the internal wiring patterns and the surface wiring patterns to one another. Therefore, the multilayer printed circuit board having the IVH structure is free from a necessity of specially forming regions for forming the through holes. Therefore, arbitrary layers can freely be connected to one another through small via holes. As a result, size reduction, high density and high-speed propagation of signals can be easily realized.
The multilayer printed circuit board having the IVH structure is manufactured by a process arranged, for example, as shown in FIG. 6.
Initially, a material having a structure in which an aramide nonwoven fabric cloth is impregnated with epoxy resin is employed as prepreg 112. Then, an operation for forming holes in the prepreg 112 is performed by using carbon dioxide gas laser. Then, conductive paste 114 is enclosed in obtained hole portions 112a (see FIG. 6(A)).
Then, copper foil 116 is laminated on each of the two sides of the prepreg 112, and then heat and pressure are applied to the prepreg 112 having the copper foil 116 by heat pressing. Hence it follows that the epoxy resin and the conductive paste of the prepreg 112 are hardened so that the electrical connection between the two copper foil members 116 on the two sides of the prepreg 112 is established (see FIG. 6(B)).
Then, the copper foil 116 on each side is patterned by an etching method so that a hard and double-sided substrate having via holes is obtained (see FIG. 6(C)).
Then, the obtained double-sided substrates are used as core layers to form a multilayer structure. Specifically, the prepreg and copper foil are sequentially laminated on the two sides of the foregoing core layer while the prepreg and the copper foil are being aligned. Then, heat pressing is again performed, and then the uppermost copper foil 116 is etched. Thus, a four-layer substrate is obtained (see FIGS. 6(D) and 6(E)). When a structure having a large number of layers is formed, the foregoing process is repeated. Thus, a six-layer substrate or an eight-layer substrate can be obtained.
The foregoing conventional technique, however, must repeat the laminating process using heat pressing and the process for patterning the copper foil by performing the etching operation. Therefore, the manufacturing process becomes too complicated and considerable time is required to complete the manufacturing operation.
If the multilayer printed circuit board having the IVH structure which can be obtained by the above-mentioned manufacturing method encounters only one defective portion (only one defective process) in the patterning operation during the manufacturing process, the overall circuit board, which is the final product, becomes a defective product Hence it follows that the manufacturing yield deteriorates excessively.
To overcome the above-mentioned problems, an object of the present invention is to provide a high-density multilayer printed circuit board having the IVH structure which can be efficiently manufactured and which permits a satisfactory high manufacturing yield.